This invention relates to viscosity index improvers and more specifically relates to the use of block copolymers containing styrene, alpha-methylstyrene (AMS), 3,4-dimethyl-alpha-methylstyrene (DMAMS) and lauryl methacrylate (LMA) for such purposes.
The need and uses of lubricant additives are described by Larson and Larson in Chapter 14 of the Standard Handbook of Lubrication Engineering, (1968), which is incorporated herein by reference. A major factor in selecting a particular lubricant system for a specific application is the lubricant's viscosity variation with temperature. This viscosity-temperature variation, designated as the viscosity index (VI) scale, is described in the ASTM test D2270 which rates the VI of an oil by measuring its viscosity at 100.degree. and 210.degree. F., and basing the index on assigned values of standard 0 and 100 VI oils. It is known that the variation in viscosity can be determined with considerable accuracy from the viscosity measurements at these two temperatures.
VI improvers are combined with oils which cannot be refined practically to a desired VI or in oils which encounter wide temperature variations such as those used in crank cases of internal combustion engines, hydraulic systems, automatic transmissions, gear cases, and air compressors. Depending on the properties of the base oil, VI values greater than 100 can be achieved most easily and economically by the use of VI improvers.
The two principal types of VI improvers commercially available are polymers of isobutylene (e.g., Exxon's Paratone) and acrylate polymers, and copolymers such as poly(lauryl methacrylate), and random copolymers of lauryl and butyl methacrylate (e.g., Rohm and Haas' Acryloid). In these additives, the molecular weight is controlled to achieve a balance between VI improver effectiveness and shear stability. This is done because, although high molecular weight polymers give high VI improvement per unit of material added, higher molecular weight polymers are increasingly subject to breakdown under high shearing conditions found in high-speed, rotating engine parts, high-speed gear cases, hydraulic systems and the like. Block copolymers containing hydrogenated blocks of isoprene and blocks of a vinyl aromatic which also may be hydrogenated are described in U.S. Pat. Nos. 3,763,044 and 3,775,329 incorporated herein by reference. These copolymers suffer from the relative costly hydrogenation procedure which must be employed in their manufacture.
The object of our invention is to provide a polymeric composition which can be used as a viscosity index improver with high shear stability. Another object is to provide a lubricant system which possesses a high viscosity index. Still another object is to provide an improved lubricant system at low cost. Other objects appear hereafter.